Effect of Temperature and Time on Nickel Aluminide Coating Deposition

The NiAl coating was deposited onto Nickel based CMSX-4 superalloy by in-situ CVD (Chemical Vapor Deposition) method. Main focus of this contribution was to study the influence of aluminizing time and temperature on the microstructure and thickness of the coating; this was followed by examination by XRD (X-Ray Diffraction), electron microscope. Results suggest that an incremental variation in temperature alters the coating activities from HA (High Activity) to LA (Low Activity). This is exhibited by the resultant CT (Coating Thickness) since HA coatings are thicker than LA counterparts. The microstructure of the coating formed at low temperature (or HA ones) showed a large amount of -Cr precipitates while one formed at high temperature (or LA ones) exhibited lower amounts of such precipitates. Moreover, incremental aluminizing time showed linear trend of CT at initial stage, thereafter (10 hrs) it leveled off. Whereas it does not affect microstructure of the coating.

T BC (Thermal Barrier Coating) is applied onto turbine blades of aeroengine as to reduce their surface temperatures and enhance engine working efficiency. TBC is a multilayered system i.e. topcoat, thermally grown oxide and BC (Bond Coat or Bond Coating). The performance of TBC is highly influenced by BC. One of the aspects to produce good quality coating is the proper control over processing parameters. Moreover, TBC is widely applied for thermal insulation of aeroengine turbine blades [1]. The ultimate goal of such system is to reduce turbine surface temperature in order to improve component life span

INTRODUCTION
and working efficiency of aeroengines. One of the most commonly adopted BCs in TBC is nickel aluminide (NiAl). It has an ordered B2 structure, high melting point, low density, and enhanced resistance to oxidation [2]. One of the key aspects to produce improved quality coatings (i.e. optimum CT and microstructure) is the control and optimization of temperature and time. Cl). However, the microstructure remained unaffected in their study [4]. Moreover, the nature of formation was suggested to be diffusion controlled [4][5][6].
On contrary, this research is to advance the knowledge of in-situ CVD parameters affecting the coating microstructure. For BC to serve in TBC system its CT plays vital role since it provides aluminum reservoir for protection. This is due to fact that the higher it is, the more is Al content that is instrumental for improved oxidation. Moreover, this is mandatory in two ways; first the coating microstructure is substrate dependent that relies on coating formation parameters and second the turbine blades thickness is ~ 1mm, thus higher CT could deteriorate super alloy mechanical properties [7][8]. Moreover, interestingly in this research it was found that the temperature could change the process activities without changing the pack chemistry.
Therefore, such behavior will be instrumental to achieve the desired CT without affecting the process assembly (in particular at lower temperature). However, at higher temperature, extended process time will be detrimental for the mechanical properties of superalloy components without significant increase in CT.

MATERIALS AND METHOD
Second generation CMSX-4 single crystal superalloy was selected for the formation of NiAl layers whose chemistry is presented in Table 1. The alloy coupons were given by Rolls Royce Plc, Derby, UK. Alloy samples into 10x10x2 and/or 10x15x2 mm discs were prepared. After sectioning, grinding was carried out using 240-1200 grit SiC papers.
Subsequently, substrates were cleaned ultrasonically and dried by using vacuum oven.
These alloy samples were coated at different times and temperatures using an in-situ CVD method. It should be noted that the main raw materials for coating deposition were the Al 2 O 3 , NH 4 Cl and Al. More details of the aluminizing conditions and procedures are reported elsewhere [9][10][11]. Once coatings were formed, a final process was applied that is called diffusion annealing at  and inner one (an IDZ (Inter-Diffusion Zone)) as shown in Fig. 1(a). Coating contains -Cr precipitates in the magnified micrograph with bright contrast as shown in Fig. 1(b) which is similar to one found elsewhere [12].

Coating Cross-Sections
Thus, it is the characteristic structure of the coating by low temperature high activity process. Line scan profile of same coating cross-section through the thickness exhibits numerous other elements in the coating i.e. Co, FIG. 1(a) 1(b). SIMILARLY THE MICROSTRUCTURE AS SHOWN ABOVE (FIG. 1(A) nature of coating formed at low temperature as shown in Fig. 1(c). This is termed as LA response i.e. coating exhibited LA nature. This will be discussed in detail in next section.
Moreover, the coating phases were also confirmed by XRD as shown earlier in Fig. 1(d) for both of these coatings. The major phase of these coatings was found to be NiAl. However, it contains the fine distribution of -Cr precipitates ( Fig. 1(d)) having overlapping peaks with NiAl phase. Similar coatings were observed elsewhere by using CVD process [13]. In general, SEM and XRD examination demonstrate that coatings deposited are characteristics of HA low temperature method [9]. Thus this clearly indicates that temperature is instrumental for the formation of kind at exactly at same pack chemistry. This mean the coating growth phenomenon was changed by the variation of temperature. This confirms the microstructural evolution of the coating as discussed earlier. Indeed, this is due to fact that the coating formed in low temperature range is commonly known to be HA [14]. Because they grow by inward and hence higher CT, whereas, at high temperature (beyond 1000 o C), the activity of Al is reduced and that of Ni is increased [2].

CT Vs Aluminizing Temperature
Hence, the resultant coating is termed as LA. Therefore, the temperature of the in-situ alters the coating growth process, hence activities of Ni and Al that are principal diffusing elements in the process [10]. It must be remembered that the HA outcome or yield is always -Ni 2 Al 3 phase. Such phase spreads or penetrates deeper into substrate and hence permitting higher CT in comparison to LA counterpart ( Fig. 1(a-b)). However, to make such phase suitable for oxidation resistance application, a diffusion annealing treatment is carried out

FIG. 2. THE CROSS-SECTION OF THE COATING DEPOSITED
AT 1100 O C WHILE KEEPING ALL PARAMETERS SAME AS ONE SHOWN EARLIER FIG. 1(a-b)

Effect of Temperature and Time on Nickel Aluminide Coating Deposition
in order to convert it into NiAl. On the contrary, LA counterpart coatings are mainly made of NiAl phase which comparably grow slower (Fig. 2). The coating activities and their thermodynamics have been excellently explained and reported elsewhere [10,14]. So, it emerges that the choice of temperature is an essential parameter to govern the growth process of coating.

CONCLUSION
In-situ CVD method was used in present study to examine the effect of aluminizing conditions on microstructure and CT. A great number of aluminide coating samples were magnificently prepared onto single crystal CMSX-4 superalloy substrates. Microstructure and CT were monitored in detail. Present results indicated significant influence of temperature on the CT and microstructure.
Progressive increase in aluminizing temperature was found to alter coating activities i.e. from HA to LA. This was exhibited by the resultant CT, as the HA coatings were thicker than that of LA counterparts. The microstructure of the coating formed at low temperature (or HA ones) contained the significant amount of -Cr precipitates while one formed at high temperature (or LA ones) showed lower amounts of such precipitates.
Moreover, at fixed temperature (880 o C); aluminizing time was found to have significant impact on the CT. However, after some time (10 hours), CT leveled off and this was attributed to the consumption of activator from the aluminizing mixture over the time. In addition, no considerable effect of aluminizing time was noticed on the coating microstructure under preset of experimental conditions. However, presumably at higher temperature, the extended time could have detrimental effect on substrate.